The present invention relates generally to an apparatus for optically detecting the presence of extraneous mater such as waterdrops on a translucent shield. The present invention may be embodied, for example, in a windshield wiper control system for motor vehicles, but will be appreciated that it is also useful in other applications.
It is known that such an optically detecting apparatus is employed to windshield wiper control for motor vehicles and is arranged so as to detect extraneous raindrops on the outer surface of the windshield by means of a sensing device comprising a light-emitting diode and a phototransistor which is located at the inner surface side thereof. An important problem in such an arrangement relates to the fact that the detection area is narrow, and in order to extend the detection area it is required to use a number of sensor devices, which causes obstruction to the driver's vision. One possible solution is that a sensor device comprising an piezoelectric element is provided on the bonnet of the motor vehicle so as to detect the vibration produced in the bonnet due to the falling of raindrops thereto. However, this raindrop sensing system arises a problem in durability because of non-protection against the weather.